Our invention relates to a gyroscopic instrument, an more particularly to a compass, of the type in which a hollow sphere containing one or more electrically driven gyroscopes mounted therein is floating in an electrically conductive liquid within the inner surface of revolution of a vessel, this surface of revolution and the outer surface of the sphere being provided with opposed electrically conductive electrode portions for the purpose of conducting the electrical current driving the gyroscopes from the vessel through the liquid to the gyroscopic motors. The vessel is provided with an inlet and an outlet at different levels and a pump is provided for circulating the liquid from the pump to the inlet, through the gap between the inner surface of the vessel and the outer surface of the sphere, through the outlet and through a passageway back to the pump. The flow of liquid so produced exerts a bearing pressure on the sphere tending to hold the center thereof on a predetermined point of the axis of the inner surface of revolution of the vessel. A gyroscopic instrument of this type forms the subject matter of U.S. Pat. No. 3,373,617 granted to Martin Lassig on Mar. 19, 1968. In all of the embodiments of the prior invention covered by this patent, the internal surface of the vessel is a spherical surface and the bearing pressure exerted by the circulating liquid on the hollow sphere containing the gyroscopes tends to keep the hollow shere in concentrical condition within the inner spherical surface of the vessel. Displacement of the center of the hollow sphere from this concentrical location causes the restoring force to be produced by the bearing pressure on the hollow sphere and this restoring force can be increased by increasing the velocity of flow of the liquid through the gap between the hollow sphere and the inner wall of the vessel. It is desirable, however, to keep this velocity as low as possible to avoid turbulence because turbulence is liable to exert a torque about one or more axes on the hollow sphere. Such torque, however, would induce highly undesirable precessional motions of the gyroscopes detrimental to the accuracy of operation of the instrument.
Therefore, it is the primary object of our invention to increase the restoring force exerted by the circulating liquid on the hollow sphere counteracting displacement thereof from the intended position of its center on the vertical axis of the internal surface of revolution of the vessel without unduly increasing the velocity of flow of the liquid through the gap. It is a more particular object of our invention to so design the gyroscopic instrument of the type indicated hereinabove that displacement of the center of the hollow sphere from its intended position on the axis of the inner surface of revolution causes substantially the same restoring force to be produced by the bearing pressure exerted by the liquid on the floating sphere irrespective of the direction of such displacement. In other words, it is a more specific object of our invention to ensure that a horizontal displacement of the sphere produces a restoring force of substantially the same power as does a displacement in vertical direction.
It is another object of our invention to so design the gyroscopic instrument of the type indicated hereinabove that a circulation of minimum velocity of flow unable to produce torques on the sphere of seriously disturbing character will yet suffice to produce bearing pressures exerting a high restoring force on the sphere in event of a displacement of the center thereof in the vessel.
In all of the embodiments of the prior invention disclosed in said U.S. Pat. No. 3,373,617 the flow of liquid through the gap is symmetrical to the vertical axis of the vessel. Since the internal surface of the vessel confining the gap is a spherical surface, the width of the gap is invariable throughout its length, when the hollow sphere assumes its intended concentrical position. The cross-section of the flow is a zone of a conical surface having a comparatively small diameter near the inlet and a much larger diameter near the equator of the internal surface of the vessel. Therefore, the cross-section increases considerably from the inlet to the equator and, as a result, the velocity of flow decreases in proportion to the increase of the cross-section. As the maximum velocity near the inlet must not exceed a rather low limit to avoid turbulence, the flow closer to the exit will be extremely slow and, therefore, unable to contribute substantially to the bearing pressure.
We have now found that the bearing pressure can be substantially increased without increasing the flow velocity near the inlet by so changing the shape of the inner surface of revolution of the vessel as to substantially reduce the change of velocity of the flow from the inlet to the equator. More particularly we so shape the vessel as to create a relatively narrow bearing zone of the gap communicating with the inlet and a relatively wider zone communicating with the outlet, the bearing zone having an exit merging said bearing zone with the wider zone, the width s of the bearing zone of the gap amounting at any point between the inlet and the exit to ##EQU1## at the level of said inlet, d.sub.o being the outer horizontal diameter of said sphere at the level of said inlet, d being the outer horizontal diameter of said sphere at said point, and K being constant amounting to from 0.9 to 1.3, s being a steady function of d between said inlet and said exit, the feeding capacity of said pump being such that the flow of the liquid through said gap from said inlet to said outlet is laminar.
Owing to this profile of the inner surface of revolution of the vessel the liquid, in spite of the very low velocity of its flow, exerts on the hollow sphere an upwardly directed bearing force sufficient to carry the overweight of the floating sphere, such bearing force adapting itself automatically to any fluctuation of the overweight as may be caused by changes of temperature. When the overweight (the difference of the weight of the hollow sphere and of the elements mounted therein minus the weight of the liquid displaced by the hollow sphere) rises thus producing a tendency of the sphere to descend, the cross-section of the bearing zone of the gap tends to decrease and this leads to an increase of the bearing pressure exerted by the circulating liquid on the floating sphere whereby the tendency of the sphere to descend is counteracted. Similarly, any lateral displacement of the sphere will produce an increase of the bearing pressure laterally of the sphere whereby the lateral displacement is counteracted.
Owing to the improved shape of the internal surface of revolution of the vessel any disturbing torques exerted by the circulating liquid on the hollow sphere are reduced to the minimum permissible in a high quality gyroscopic instrument such as a compass. Irrespective of the low velocity of circulation of the liquid the floating sphere is effectively held within the vessel in its intended position even where displacing forces, such as acceleration, act on the floating sphere thus preventing any contact of the sphere with the vessel. More particularly, horizontal acceleration of the gyroscopic instrument will not produce excessive horizontal displacements of the floating sphere. Where the floating sphere has a weight of 8.5 kp, disturbing torques acting on the sphere are reduced by our invention to less than 0.01 cm .p. When applied to a compass, our invention will guarantee an accuracy of indication of the azimuth of .+-.0.1.degree.. When our invention is applied to smaller gyroscopic instruments in which the weight of the floating sphere is lower, the permissible disturbing torques are smaller in the relation of 1:100.
In a preferred embodiment of our invention the width of the gap at any point of the bearing zone thereof amounts to 0.013 - 0.0075 of the diameter of the sphere but at least to 0.5 mm.
Preferably, our invention is applicable to the type of instrument illustrated in FIG. 3 of the U.S. Pat. No. 3,373,617 referred to hereinabove in which the gap between the floating sphere and the internal walls of the vessel has a relatively narrow bearing zone communicating with the inlet and a relatively wider zone communicating with the outlet.
Further object of our invention and the advance attained thereby will appear from the detailed description of various embodiments thereof described hereinafter with reference to the drawings. It is to be understood, however, that our invention is no way restricted to such embodiment but is capable of numerous modifications within the scope of the appended claims.